Arrangement for presses

ABSTRACT

A press operating with hydraulic pressure comprises a pair of relatively movable components (8, 11) which during a pressing operation carry out a relative movement which is sensed. By use of the sensed relative movement a control signal is generated which is used for regulating the hydraulic pressure so that this is caused to vary with the relative movement according to a certain functional relationship. For generating the control signal a manually programmable electrical function generator (23) is provided, preferably a panel (29) comprising plug contacts (34), on which a plurality of pairwise associated pressure and movement values, constituting electrically and visually readable points in the desired functional relationship, can be set up. To the input (28) of the function generator a measurement signal is fed which is a measure of the relative movement covered by the two components and which makes the function generator read off and issue at its output (41-43) the pressure values as the set movement values corresponding thereto are reached.

TECHNICAL FIELD

The present invention relates to an arrangement for presses particularlythose intended for forming sheet metal blanks and wherein forming iseffected by hydraulic pressure which varies during a pressing operation.Presses of the type here under consideration comprise a pair ofrelatively movable components which are made to carry out a relativemovement during the pressing operation, means for sensing the saidmovement and generating a control signal which, according to afunctional relationship relevant to the pressing operation, depends onthe second movement, and a hydraulic system for regulating the hydraulicpressure by means of the central signal so that during the pressingoperation hydraulic varies according to the functional relationship.More specifically, the invention relates to an improved arrangement forcontrolling the hydraulic working pressure which acts in such a pressduring a pressing operation and which thereby controls, either directlyor indirectly, the course of the operation.

THE PRIOR ART

Pressure control of this kind in a press is necessary so thatsatisfactory working results may be obtained when carrying out deepdrawing and similar pressing operations when sheet metal blanks are tobe given a complicated form. For this purpose, various solutions may befound on the market, based on drawing a curve which represents on aspecific scale the relationship between the hydraulic pressure and themovements carried out, which relationship has been found to be suitablefor a pressing operation. A templet is then produced from plate or otherrigid material to have a contour that corresponds accurately with thecurve, and this templet is fixed to a movable means appertaining to thepress which mechanically detects the movements involved. A templetsensor, the sensing part of which follows the contour of the templet andis movable in the lateral direction relative to the displacementdirection of the templet, during the pressing operation transmits itslateral movements, obtained from the displacements of the templet to asignal generator in the form of a pressure control valve or apotentiometer which controls an electro-hydraulic valve via anamplifier. The latter arrangement is described in more detail in U.S.Pat. No. 3,962,895.

To establish the relationship between the relative movement and theworking pressure which the contour of the templet represents generallycalls for a very comprehensive sampling process, particularly for partswith high drawing depth ratio, and it is not unusual before startingseries-production for 20 samples to have to be made, with adjustments inbetween at different points over the contour of the templet before asatisfactory work-piece is obtained. Obviously, a templet cut out ofplate is less suitable, but even if a cam plate is used, the contour ofwhich is formed by a steel strip which is adjustable by means of anumber of positioning screws connected to the steel strip, suchadjustment work is still time-consuming and troublesome and cannot becarried out without tools.

Furthermore, it is difficult to carry out a purely mechanicaltemplet-sensing which is very accurate without the transmitted movementvarying from one run to the next due to mechanical imperfections in thesignal generating control valve.

A templet-sensing arrangement for hydraulic presses which works withphotocells is known from Swedish Pat. No. 322 193. The templet which, asin the above-described arrangement, is displaced by one of the movingcomponents of the press, can be made in this instance from a piece ofpaper which is cut so that it gives the desired pressure curve and whichis then inserted in a holder so that depending on its movement, lightfrom a light source located on one side of the templet is screened fromor reaches a row of photocells arranged on the other side. Thearrangement requires that every time a new part is to be produced atemplet must be made, or the pressure curve adjusted on an existing oneif it is not desired to file all the prepared templets for future use,and the procedure for trimming a pressure curve into shape is basicallyjust as labour and time-consuming as with a conventional templet made ofplate, for in this case too the templet must be removed from the machinefor each modification of the pressure values, and after adjustment,which can be effected by cutting away or sticking on a piece of paperalong the contour of the pattern, it must be inserted again in its placein the holder.

With the known pressure regulating arrangements there is also noflexibility during series-production, as might be described foraccommodating the variations which may arise concerning the hardness ofthe basic material or other characteristics which are significant forthe choice of working pressure. Neither does any one of the priorarrangements allow a simple and quick adjustment of the pressure curvein order to compensate for temperature differences in the pressuremedium while pressing is going on, or for the rises or reductions inpressure occasioned thereby, which in certain circumstances can resultin an interruption of work several times a day.

THE OBJECT OF THE INVENTION

It is therefore the object of the present invention to provide apressure-regulating arrangement for use in presses which operate withhydraulic pressure that is to be varied during a pressing operation,which arrangement is not encumbered with the disadvantages that theknown arrangements display, as mentioned above, but enables thefunctional relationship which should exist during a pressing operationbetween the hydraulic pressure and the forming movement to be programmedinto the press quickly and with few manipulations, and has suchflexibility that a relationship inserted the press will be simple toalter and adjust subsequently without any appreciable interruption in acontinuous production process.

Another aim which the invention seeks to achieve is to generate thecontrol signal which represents the working pressure in a way that isnot subject to errors due to inaccurate movement sensing, mechanicalwear, etc., and whereby the course of a pressing operation can befollowed visually, so that the movement and pressure values prevailingat any given moment according to the relevant relationship can beobserved, and the latter can be reproduced for filing, when applied,without preventing the same relationship being used in the productionprocess.

THE SOLUTION

This is accomplished according to the primary characteristic of theinvention in that the said means comprise a manually programmableelectric function generator which has a large number of contact pointsarranged in rows and columns for reproducing in electrically readableform pairwise associated pressure and movement values constitutingpoints in the relevant functional relationship and which at its outputis connected to the hydraulic system, while its input side isoperatively connected via an electrical measurement transducer to eitherof the said relatively movable components of the press so that thefunction generator receives a measurement signal which is a measure ofthe movement covered and which makes it in turn and in sequence read offand produce at its output the said pressure values as the movementvalues are reached.

The function generator consists preferably of a programming panel, theelectrical parts of which comprise in a known way two crossing systemsof parallel conductors situated in different planes, which areseparately assigned to the pressure or movement values respectively andwhich increase stepwise and gradually within each system, from a commonstarting point. The panel also has in a known way a perforated boardcovering the conductor systems, the holes in which are located right atthe intersection points of the system, and suitable plug contacts forfitting in the holes.

For programming the pressure and movement relationship, the procedure isto insert, for each of the movement values, a plug contact in the holewhich corresponds with a selected demand pressure value so that the plugcontact positions on the panel depict as a whole the pressure curvebelieved to be suitable for a certain pressing operation. If the workingresults are not satisfactory it is possible with a few manipulations toadjust the curve rapidly, by moving one or more of the plugs so that thedemand value of the hydraulic pressure for a specific relative positionof the movable components of the press, or during a certain part of thetotal forming movement, is raised or lowered compared with the formervalue.

Each functional relationship programmed in this way can be recordedeasily and rapidly for filing by providing the programming panel with adiagram sheet which is pierced by the plug contacts as they are insertedinto the holes in the panel.

It is also of great practical importance that, according to a furthercharacteristic of the invention, the function generator has a row oflamps or the like which are lit in turn, in time with the sensedmovement values. In this way, the operator can easily check the progressof a pressing operation.

DESCRIPTION OF A PREFERRED EMBODIMENT

The invention will be explained in more detail in the following withreference to the attached drawing, in which

FIG. 1 is an axial section through a hydraulic press with thearrangement according to the invention,

FIG. 2 is an electrical circuit diagram of the arrangement, and

FIG. 3 illustrates a programmed relationship between the formingmovements and the pressure.

The invention can be used on any type of press in which forming dependson hydraulic pressure that is controlled during the forming process and,and thus also on a hydraulically driven press wherein the forming partworks mechanically, and likewise in a press which is driven purelymechanically and has a hydraulic forming chamber in which the hydraulicpressure which is effective during the forming process is controlled byaltering the volume of the chamber. However, for the sake of simplicitythe invention is here described only in its application to a hydraulicpress constructed according to FIG. 1 in 1 in Swedish Pat. No. 7316381-8(Publication No. 395.391) corresponding to U.S. Pat. No. 3,962,895 andreference should be made thereto concerning any details which are notdescribed below.

In FIG. 1, 1 designates a stand which bears at its top a unit 2 which isspecially designed for forming sheet metal blanks 3 and comprises amembrane 4 which defines the bottom of a closed chamber 5 containing aconstant volume of fluid.

On its lower section the stand 1 bears a hydraulic assembly 6 comprisinga cylinder 7 in which an outer pressing piston 8 is inserted, whichextends at its top, via an intermediate base 9, into a draw ring 10 onwhich the sheet metal blank 3 is placed when the press is in the openposition. Running in the outer piston 8, the hydraulic assembly also hasan inner piston 11 which bears a central tool component, in thisinstance constructed as a die 12. The chambers 13 and 14 underneath theouter and inner pistons, respectively, are filled with hydraulic fluidand are connected via pressure lines 15 and 16 to a hydraulic systemcomprising a pump assembly 17. The hydraulic system is designed toeffect all the fluid transportation required for closing the press asthe chamber 13 is contracting, whereby during the expansion of the fluidchamber 13 the outer piston 8 moves upwards into the position shown inthe Figure, bringing the inner piston 11 with it; similarly, for openingthe press, pressure medium is introduced into the two annular chambers18 and 19 which are respectively located between the cylinder 7 and theouter piston, and between the outer piston and the inner piston.

The forming operation itself is effected, as is known in this kind ofpress, in that the inner piston 11 together with the die 12 is made tomove upwards by the supply of pressure fluid from the pumping assembly17 via the line 16 into the chamber 14, so that the sheet metal blank 3is drawn over the die and deformed. During this process a high fluidpressure arises in the forming chamber 5, which leads to the result thatunder the influence of the membrane 4 the sheet metal blank is pressedagainst all sides of the die and assumes the shape thereof, and inaddition the casing of the unit 2, the draw ring 10 which rests thereonand the outer piston 8 move downwards while fluid is controlled bydrained from the chamber 13 via the line 15 to the hydraulic system. Asexplained in Swedish Pat. No. 7316381-8 (Publication No. 395.391corresponding to U.S. Pat. No. 3,962,895), with so-calledmatrix-forming, these relative movements of the two tool components arethe reverse of what is described here.

With this type of pressing operation the specific pressure in theforming chamber 5 is of great importance for the working results and ithas been found that this pressure should be controlled so that it variesin a specific way with the mutual movement of the tool components, thatis, as a function of the relative movements between the outer piston 8and the inner piston 11, which parts are hereinafter called the twomoveable components of the press. One way proposed to accomplish thispressure control is to allow the fluid which is drained from thehydraulic assembly 6 during forming to pass through a valve arrangement20 which ensures that the pressure upstream from the valve arrangementand with it the pressure in the fluid chamber which contracts duringforming (i.e. the chamber 13 in the example of die-forming) correspondsto a demand value which varies in dependence upon the mutualdisplacement of the two movable components of the press. Accordingly,the valve arrangement 20 indirectly controls the fluid pressure which iseffective in the forming chamber 5. The fluid drained off in this waycan either go to a tank 21 or, as described in U.S. Pat. No. 3,962,895,it can be returned to the fluid chamber which is expanding (in theexample this is chamber 14) so as to cooperate in this way in the workwhich the upwards-moving tool component is carrying out, therebyreducing the consumption of energy.

Thus, the hydraulic pressure which controls the forming is either thepressure which acts directly on the blank during the forming process orthe indirectly acting pressure p which, in a press of the kind describedabove, prevails in a fluid chamber appertaining to a hydraulic assemblyand compressed during the forming process. According to the presentinvention, that hydraulic pressure is regulated with the aid of anarrangement which consists basically of a measurement transducer 22 forsensing the mutual displacement s of the two movable components 8 and 11of the press, and an electrical function generator 23 for manualprogramming of the functional relationship p=f(s) between the saidpressure p and the displacement s that is to apply during a formingoperation and whereby, according to the development of the senseddisplacement movement, a control signal 15 is produced which variesaccording to the programmed relationship and is supplied to the valvearrangement 20.

The measurement transducer 22, is assumed to be the type of opticalabsolute position transducer which produces a digitally codedmeasurement signal. It comprises a grid disc 24 which is attachedrotatably but not displaceably to the intermediate base 9, and a rod 25which rotates the grid disc and which is attached to the inner piston 11by means of a bracket 26. The bracket 26 projects out laterally througha slit 27 in the draw ring 10 that has a vertical extent such that thebracket can move freely relative to the draw ring during the upwardsdisplacement of the inner piston. The grid disc 24 thereby rotatesthrough an angle which exactly corresponds to the mutual displacementbetween the two moving components 8 and 11 and which is unambignouslyrepresented by the digital measurement signal of the transducer 22 whichis supplied to the input 28 of the function generator. There is nothingto prevent an analogue position transducer, such as, a contact-freepotentiometer, from being used instead, in which case another adaptationmay be effected between the position transducer 22 and the functiongenerator.

The function generator, the electrical structure of which is shown inFIG. 2, comprises advantageously a conventional programming panel 29with two conducting systems which cross each other and which are locatedin separate planes so that the conductors themselves do not provide anylink between the systems. Each system comprises a large number ofequidistantly located conductors (of which only a small number are shownin the drawing and the two systems), together form a regular checkedpattern, the rows 30 and columns 31 of which represent pressure andmovement values respectively, all gradually increasing in small stepsfrom the lower left-hand corner 32 of the panel.

As shown here, the conductors systems are covered by a perforated sheet33 which is made of insulating material and in which there are holeslocated right at the intersection points. By fitting a plug contact 34in a hole, the two row and column conductors which pass nearest it areconnected to each other, thus allowing a point in a desired functionrelationship to be programmed in thefunction generator. On the board,the function values corresponding to the positions of the holes shouldbe given, the pressure values expediently in percent of the maximumoperating pressure of the press, and displacement values in appropriateunits of length. The programming panel is also provided with anilluminated array 35, the lamps 36 of which are associated one by onewith the columns 31 of the panel and are connected to a decoder 37 whichreceives measurement signals from the position transducer 22 and whichis designed to light the lamps 36 in turn and in sequence as themovement values corresponding to the columns 31 are attained.

At the input 28 the function generator has a second decoder 38 which isconnected to each of the column conductors 31 via a gate 39, and whichis designed so that, during a forming operation, it activates one of thecolumn conductors 31 with the aid of the measurement signal received,which will occur in the same sequence and at the same moment that thecorresponding lamp 36 is lit. This activation can be effected so thatthe conductor in question is connected to electrical earth by means ofits gate while all the remaining column conductors remain inactive dueto the fact that their gates have a high impedance to earth.

The output side of the function generator comprises a resistance chain40 in which each separate resistance is connected between a pair ofadjacent conductors in the rows 30, and has a resistance value such thatthe series resistance for the chain increases, preferably by the sameamount at each step and calculated from the terminal point 41 accordingto the pressure values appertaining to the rows. A current generator 42connected to the terminal point provides a current of constant value andwhile forming is going on, this current can be conducted to earth onlyvia the column conductor 31 which is activated at that moment. Assumingthat this is the conductor 31 which is connected to the plug contact 34,the current will accordingly pass through the two lower resistances inthe chain 40, then via the horizontal conductor 30 that is engaged bythe contact plug, and then via said conductor 31 and its gate to earth,while at the same time all the other paths through the functiongenerator are interrupted. The potential at the point 41 is graduallychanged accordingly, in time with the forming movement and by the amountwhich obviously depends on the row in which the plug contact isinserted.

A voltage with such a characteristic is not, however, used directly forcontrolling the pressure, but is first modified, according to acharacteristic feature of the invention, into a continuously varyingsignal, this being effected in a filter 43. This can consist of a RCcircuit with a variable condenser, and by using this to adjust thefilter time-constant in relation to the prevailing relative speed of themoving components in the press, it is possible to smooth, at least to alarge extent, the stepwise variation in the direct current produced inthe function generator at the transition from one column 31 to another.

If, as in the example shown, the valve arrangement 20 comprises apressure regulating valve 44 the characteristic of which is such thatthe pressure is proportional to the current in a spool 45 in the valve,it can be expedient to process the filtered direct current signal in thefollowing way. In an oscillator 46, an alternating current is producedwhich has a small amplitude compared with the direct current and whichis superimposed on the latter in an operational amplifier 47 so that, ina known way, the mechanical hysteresis of the pressure control valve, orits tendency to stick, is eliminated. The summing signal, the meancurrent value of which corresponds to the current at the point 41, isapplied to another operational amplifier 48 where it is compared withthe voltage across a low resistance resistor 49 that is,series-connected to the valve spool 45, this voltage, which is thereforea measure of the valve current, being fed back by means of the conductor50. The resulting control current is supplied finally to an amplifier 51which forms the drive for the valve spool. The current to this willtherefore be proportional to the current at the input of the amplifierand therefore to the potential at the point 41.

In applications where the pressure control calls for large amounts ofthrough-flow, which is the case with presses working in the same way asor in a similar way to the example in FIG. 1, the pressure control valveis operated as a pilot valve controlling a main valve 52. This isdimensioned in such a way that it is capable of taking over all or mostof the stream of fluid which is to be drained from the press into thetank 21 or is to circulate from one fluid chamber 13 in the press,through the valve to the other fluid chamber 14.

It will be seen that with the described arrangement an effective andexpedient pressure control is obtained, which follows accurately a givenrelationship p=f(s) which, by means of the arrangement, can be variedwithin wide limits according to the type of pressing work, and can bemodified or adjusted easily and quickly during continuous production. Afurther advantage which characterises the arrangement according to theinvention is that such a relationship can be reproduced with simplemeans in its form as inserted in the function generator. For thispurpose, the programming panel is provided with a sheet of diagram paper53 (see FIG. 3) which can be provided with printed fitting marks 54matching the corners of the panel and/or with a checked pattern 55corresponding to the pattern of holes in the board 33. By means ofholders which are not shown, the diagram paper is attached in thecorrect position on the board after which the actual pressure curve istransferred to the paper pressing by the plug contacts 34 through thepaper so that they make a series of holes 56 corresponding with theprogrammed points. In comparison with the filing of templets which aresensed mechanically or optically, the method affords the advantage thatthe pressure curve of a certain work-piece can be filed in easilyhandled form, or can be used for preparing a subsequent operation whilethe work-piece involved is in production.

We claim:
 1. Apparatus for a sheet metal forming press wherein there isat least one component which moves in relation to another part of thepress during a forming operation and is controlled in its movements byhydraulic fluid under pressure, said apparatus providing for control ofthe pressure of said hydraulic fluid in accordance with a predeterminedbut readily alterable program of relationships between position of saidcomponent and magnitude of said pressure, and said apparatus comprisinga position transducer that produces a position output which varies independence upon the position of said component and pressure regulatingmeans for changing the magnitude of said pressure in correspondence withchanges in value of a characteristic of an electrical control currentfed to said pressure regulating means, said apparatus beingcharacterized by:(A) a plurality of elongated, laterally spaced apartposition conductors, one for each of a plurality of positions of saidcomponent, all extending substantially in one direction; (B) a pluralityof elongated, laterally spaced apart pressure value conductors, one foreach of a plurality of magnitudes of said pressure and each extendingtransversely to all of said position conductors but out of contact withthem to be electrically connectable to any selected one of said positionconductors by means of a readily disconnectable connector; (C) inputcurrent means normally connected with all of said pressure valueconductors for feeding an input current to all of them that has amagnitude for each of them which is different from that of the inputcurrent fed to the others and which, for each, corresponds to a uniquemagnitude of said pressure; (D) circuit control means comprising:(1) adecoder connected with said position transducer to receive said positionoutput therefrom, and (2) means under the control of said decoder forestablishing each of said position conductors selectably andalternatively in a state of connection with said pressure regulatingmeans or in a state of disconnection therefrom and whereby the positionconductor for the existing position of said component is established inone of said states and all of the others are maintained in the other ofsaid states; and (E) electrical circuit means for so connecting all ofsaid pressure value conductors with said pressure regulating means as tofeed to the latter a control current having a value that substantiallycorresponds to the magnitude of the input current fed to a pressurevalue conductor connected with the position conductor that is in itssaid one state.
 2. The apparatus of claim 1 wherein said input currentmeans comprises a plurality of resistors, one for each pressure valueconductor, each connected between its pressure value conductor and anadjacent one, said resistors being connected in series with one anotherand a source of current so that said magnitude of the input currentdiffers stepwise from one to another of the pressure value conductors.3. The apparatus of claim 1, further characterized by:(F) a plurality ofsignaling devices, one for each of said position conductors; and (G)means for energizing each signaling device, to cause it to issue aperceptible signal, whenever its position conductor is in its said onestate.
 4. The apparatus of claim 1 wherein all of said positionconductors are contained substantially in one plane and extendsubstantially parallel to one another and wherein all of said pressurevalue conductors extend substantially transversely to said positionconductors and are substantially contained in another plane that isspaced from and substantially parallel to said one plane, furthercharacterized by:(F) a panel substantially contained in a third planethat is spaced from and substantially parallel to the first twomentioned planes and which has holes for removably receiving aconducting plug that comprises said connector, said holes being soarranged as to enable any selected position conductor to be connectedwith any selected pressure value conductor by means of said plug.
 5. Theapparatus of claim 1 wherein said electrical circuit means comprises acapacitance whereby said value of the control current is caused tochange gradually but in substantial conformity to stepwise changes insaid magnitude of the input current fed to pressure value connectorsconnected to position conductors that are successively converted to saidone state.